Support assignment of capabilities to C registers

Support assignment of capabilities to C registers

Enable assignment of capabilities to C registers while preserving
the capability tag. This enables operations like the following:

set $c0=$c1
set $c0=p, where "p" is a capability (pointer) in AARCH64-CAP

Due to the X/C register overlap, we also force a re-read of register
data after every register write. So any 'G' packets are immediately
followed by a 'g' packet.